Semiconductor die-based image sensors are well known to those having skill in the electronics/photonics art and, in a miniaturized configuration, are useful for capturing electromagnetic radiation (e.g., visual, IR or UV) information in digital cameras, personal digital assistants (PDA), internet appliances, cell phones, test equipment, and the like, for viewing, further processing or both. For commercial use in the aforementioned extremely competitive markets, image sensor packages must be very small. For some applications, a package of a size on the order of the semiconductor die or chip itself, or a so-called “chip-scale” package, is desirable if not a requirement.
While traditional semiconductor devices, such as processors and memory, are conventionally packaged in an opaque protective material, image sensors typically comprise a light wavelength frequency radiation-sensitive integrated circuit (also termed an “optically sensitive” circuit or region, or “imaging area”) fabricated on the active surface of a semiconductor die and covered by an optically transmissive element, wherein the optically sensitive circuit of the image sensor is positioned to receive light radiation from an external source through the optically transmissive element. Thus, one surface of the image sensor package conventionally comprises a transparent portion, which usually is a lid of light-transmitting glass or plastic. For photographic or other purposes requiring high resolution, the chip is positioned to receive focused radiation from an optical lens associated therewith. The image sensor may be one of a charge-coupling device (CCD) or a complementary metal oxide semiconductor (CMOS). The optically sensitive circuit of each such sensor conventionally includes an array of pixels containing photo sensors in the form of photogates, phototransistors or photodiodes, commonly termed an “imager array.”
When an image is focused on the imager array, light corresponding to the image is directed to the pixels. An imager array of pixels may include a micro-lens array that includes a convex micro-lens for each pixel. Each micro-lens may be used to direct incoming light through a circuitry region of the corresponding pixel to the photo sensor region, increasing the amount of light reaching the photo sensor and increasing the fill factor of the pixels. Micro-lenses may also be used to intensify illuminating light from pixels of a non-luminescent display device (such as a liquid crystal display device) to increase the brightness of the display, to form an image to be printed in a liquid crystal or light emitting diode printer, or even to provide focusing for coupling a luminescent device or receptive device to an optical fiber.
Various factors are considered in the design and manufacture of image sensor packages. For example, the extent to which a large number of packages can be at least partially, if not completely, fabricated simultaneously at the wafer level is a substantial cost consideration. Furthermore, if the package design or fabrication approach, even if conducted at the wafer level, necessitates that all of the image sensor semiconductor dice located thereon be packaged regardless of whether a significant number of the dice are defective, a substantial waste of materials results. Also, the package lenses must be carefully positioned relative to the optically sensitive circuit on each of the dice to achieve uniformly high quality imaging while precluding entry of moisture and other contaminants into the chamber defined between the optically sensitive circuitry and the lens.
One significant problem with conventional lens positioning techniques arises when an imaging area comprising a pixel array is located non-symmetrically on an image sensor die, whereas conventional lenses are configured so that the optical center of the lens is symmetrically, or centrally, positioned over the image sensor as a whole, and not over the pixel array. The non-symmetrical location of the pixel array is often dictated by integrated circuit design constraints imposed by locations of circuits, for example memory, and bond pads on the active surface of the image sensor die. In other words, as a result of the circuit designers' attempts to optimize the electrical aspects of an image sensor die, the pixel array becomes positioned off-center.
Thus, there remains a need for a packaging technique which accommodates symmetrical non-positioning of the imaging area on an image sensor, which technique may be effected at a wafer level and which provides high quality image sensor packages incorporating a non-symmetrical imaging area.